1. Field of the Invention
This invention relates to a correction and/or detection device for product code block data applied to e.g. a so-called D1 or D2 format digital video tape recorder.
2. Description of the Prior Art
The formats for digital video tape recorders for recording video signals in a digital form include a so-called D1 format in which component signals of luminance and chrominance signals are digitized, and a so-called D2 format in which composite signals of the standard television systems, such as NTSC or PAL systems, are digitized directly. With the digital video tape recorder of the D1 or D2 formats, a product code system is adopted which enables a high error correcting ability by using inner and outer codes.
With the above D2 format digital video tape recorder, 8-bit-per-sample video data are handled in, for example, the NTSC system, in which video data have been sampled at the sampling frequency fs which is four times the subcarrier frequency fsc, or 4fsc. Referring to FIG. 1, one field is composed of 255 effective lines, with each line containing 768 samples of effective data, with the exclusion of horizontal sync signal portions. Thus the above video data are 768.times.255.times.8 bit data, with the exclusion of vertical sync signal portions. These video data are alternately allocated on a sample-by-sample basis to two channels, that is a channel 0 and a channel 1, while the 255 lines are divided into three equal segments 1, 2 and 3, each consisting of 85 lines. The video data are recorded and/or reproduced via six video tracks, as shown in FIG. 2.
With the D2 format digital video tape recorder, segment data are distributed to the respective channels, that is 768/2.times.85 byte data, are distributed, after shuffling, to six blocks each consisting of 64.times.85 bytes, as shown in FIG. 3A. Then, 4-byte outer codes for error correction and checking are annexed to each vertically extending 64-byte outer code block, as shown in FIG. 3B, and then 8-byte inner codes for error correction and checking are annexed to each horizontally extending 85-byte inner code block, as shown in FIG. 3C. Double error correction and checking may be realized in this manner by adopting the Reed-Solomon code with the inner codes (93, 85) and the outer codes (68, 64).
Six product code blocks, each composed of the annexed outer and inner codes and the data, constitute one video track, as shown in FIG. 3C, and the 85-line video data are recorded on the two video tracks, that is the channel 0 and channel 1 video tracks.
The manner in which recorded data are reproduced and the manner in which error correction and checking is performed by the product code will be hereinafter explained.
The error correction checking is performed with each product code block as a unit. Correction by the inner code is first made. Thus, correction with the inner code block as a unit is performed, as shown in FIG. 4A.
With the correction by the inner code, error correction can be made satisfactorily if the number of errors in one inner code block is up to three, as shown in FIGS. 4B and 4C. However, when the number of the errors in one inner code block is four or more, error correction cannot be made satisfactorily, and it is only detected that there are four or more errors in the inner code block. In this manner, error correction is made on all of the inner code blocks but, if error correction can not to be made, error detection is performed.
After the error correction by the inner code is terminated, error correction by the outer code is performed for correcting the portion which was unable to be corrected by the inner codes. This correction is performed with an outer code block shown in FIG. 6A as the correction unit. When the number of the regions which were unable to be corrected by the preceding correction by the inner code and which were detected as the regions containing the error is up to four within an outer code block as shown in FIG. 6B, complete error correction is made with the use of the results of detection obtained by the inner code correction. In this manner, all video data are corrected, as shown in FIG. 6c. To perform an outer code correction by employing the data of the error region, that is, the error flags obtained by the inner code correction, is known as erasure correction.
If, as a result of the correction by the inner code, five or more error flags indicating the detection of error regions are set, as shown in FIG. 7A, error correction by solely the outer code is performed, without having recourse to the results of error detection by the inner codes. Only the outer code blocks which have been detected as being free of errors are ultimately output as correct data.
When the video data are allocated to six blocks shown in FIG. 3, as described previously, the video data are scrambled by a prior shuffling, so that, even if several product code blocks are all in error, video data can be reconstituted by interpolation.
Meanwhile, when a so-called insert edition is performed on a prerecorded magnetic tape in a digital video tape recorder of the D1 or D2 format in which plural product code blocks are comprised in one track and the inner codes of the product codes are recorded on an end of the track, new data are recorded by overwriting old data without using an erasure head. In case of a failure in overwriting due to, for example, head clogging old data are partially left as unerased data. Such failure in overwriting due to head clogging, for example, occurs for a longer period of time in the form of a burst such that the failure occurs across plural inner code correction blocks, as shown in FIG. 8A.
When the correction by the inner code is first made in this case, new data and old data exist together in the inner code blocks in which the start and end points of the unerased old data portions are located as in the example of FIG. 8A. Supposing that four or more data errors exist in the unerased data portions, error correction becomes infeasible, so that the error flag indicating the presence of four or more errors in the inner code block is output as the result of detection.
On the other hand, old data of the unerased portions are correct data per se, so that they cannot be detected as errors by the inner codes and hence are output in admixture with new data. In the conventional reproducing system, full trust was placed on the error flags of the inner codes and the outer code correction was performed only on the regions where the error flags were set in decoding the reproduced data. Thus the unerased old data portions where no error flags have been set in the course of the inner code correction, as shown in FIG. 8B, are directly output as error-free data.
When the data that have undergone the correction by the inner codes as shown in FIG. 8B are to be corrected by the outer codes, the error correction is made with the use of old data which are intrinsically error data, so that, as shown in FIG. 8c, correct data are handled as error data and mistakenly corrected.
Such malfunction occurs not only on the occasion of failure in erasure but also on the occasion of mistaken error detection or correction during correction by the inner codes.
In a digital audio tape (DAT) system, since the corrective capacity of the inner codes is fully utilized, a strategy is used in which the reliability of the error flags of the inner codes is checked by the outer codes or in which correction of the regions where no error flags are set is made by the outer codes. Although such error correction and detection is made solely by the outer codes and hence the detection probability is low, such low detection probability raises no particular problem since the mistaken correction occurs on only rare occasions and such mistaken correction, should it occur, is limited to one of the inner code blocks or to several samples in one inner code block.
However, since the failure in erasure occurs more frequently than the mistaken correction and since several inner codes may fail to be detected, the detection errors are likely to be produced unless the detection probability is raised. On the occurrence of an erasure, burst errors are likely to be produced in its vicinity. It is therefore more desirable that correction or detection of unerased data portions can be made even if the number of the error flags is increased to a more or less extent. In addition, if detection cannot be made because of the presence of an excessive number of error flags, unerased data portions would be directly output unless means are provided for detecting if there are any unerased data portions in the product code block. The above mentioned DAT system is primarily designed for error correction and, should the unerased data be detected in an error-free condition, the unerased data would be reproduced as the output.
Meanwhile, with the conventional D2 format digital video tape recorder making use of the Reed Solomon code with the inner codes (93, 85) and the outer codes (68, 64), the data of the unerased portions may or may not be detected as error depending on whether the number of errors in the product code block is not more than three or not less than four, respectively.